A conventional ultrasound imaging system comprises an array of ultrasonic transducer elements for transmitting an ultrasound beam and receiving a reflected beam from the object being studied. By selecting the time delay (or phase) and amplitude of the applied voltages, the individual transducer elements can be controlled to produce ultrasonic waves which combine to form a net ultrasonic wave that travels along a preferred vector direction and is focused at a selected point along the beam. Multiple firings may be used to acquire data representing the same anatomical information. The beamforming parameters of each of the firings may be varied to provide a change in maximum focus or otherwise change the content of the received data for each firing, e.g., by transmitting successive beams along the same scan line with the focal point of each beam being shifted relative to the focal point of the previous beam. By changing the phase rotation and amplitude of the applied voltages, the beam with its focal point can be moved in a plane to scan the object.
The same principles apply when the transducer array is employed to receive the reflected sound energy. The voltages produced at the receiving elements are summed so that the net signal is indicative of the ultrasound reflected from a single focal point in the object. As with the transmission mode, this focused reception of the ultrasonic energy is achieved by imparting a separate phase and gain to the signal from each receiving element.
Many conventional ultrasound imaging systems have included a two-dimensional transducer array (hereinafter a 2D transducer array). For purposes of this disclosure, a 2D transducer array is defined to include a transducer array where the center points of the transducer elements form a two-dimensional pattern. The two-dimensional pattern may follow a curved surface according to some embodiments. Typically, the transducer elements are dimensionally similar in both length and width in a 2D transducer array. Also, a 2D transducer array may have full electronic focusing and steering. The 2D transducer array typically comprises a number of transducer elements arranged in a grid; the grid may have a square, rectangular, hexagonal, or other basis. By controlling the timing and amplitude of the elements in the 2D transducer array, it is possible to steer the transmitted ultrasound beam simultaneously in both an azimuth direction and in an elevation direction. The use of a 2D transducer array allows the ultrasound transducer or probe to have greater flexibility.
It is often advantageous to collect ultrasound data using a continuous-wave Doppler imaging mode for the imaging of moving fluids such as blood. In a continuous-wave Doppler imaging mode, a group of transducer elements in a transmit aperture are used to transmit ultrasonic energy into the anatomical region being imaged. A second group of transducer elements in a separate receive aperture are used to detect reflected ultrasonic energy that is reflected back from the region being imaged. Typically, for a probe with a 1D transducer array, there is an amplifier associated with each of the transducer elements in the receive aperture. However, due to both space and power constraints, the amplifiers within a probe with a 2D transducer array typically have limited dynamic range, leaving them poorly-suited for procedures such as continuous-wave Doppler imaging. In continuous-wave Doppler imaging, the signals from moving blood may be orders of magnitude weaker than the signals from stationary tissue. For proper signal analysis, both the weak blood echoes and the strong tissue echoes must be processed. Using amplifiers with limited dynamic range, such those which would be found in a conventional probe with a 2D transducer array, to process the signals received during a continuous-wave Doppler acquisition may be problematic since the blood echo signals are so much smaller than the signals received from tissue. Generally, the use of amplifiers with a limited dynamic range for continuous-wave Doppler imaging will result in sub-optimal analysis of the blood echoes.
Additionally, 2D transducer arrays typically require electronic beamforming in close proximity to the transducer array for optimal results. However, conventional 2D arrays may have several thousand elements and it is not practical to bring electrical signals from all of these elements back to the ultrasound console for electronic beamforming.
For these and other reasons there is a need for a new ultrasound probe and a new ultrasound imaging system.